Of available DC-DC converters used for power conversion purposes, the flyback converter is the most simple. Its minimum configuration consists of only a switch, a transformer, a diode and two capacitors (one at the input port and the second one at the output port). In low-output-voltage converters, the conduction loss of the diode rectifier due to its forward voltage drop becomes the dominant power loss. In certain conditions this loss can reach 50% of the total power loss. The simple approach conceived to reduce the above-mentioned power loss was to replace the rectifying diode with a synchronous rectifier, i.e. with a low-ON-resistance MOSFET. To perform the normal operation of the flyback converter the control of the MOSFET used as the synchronous rectifier is easily obtained by inverting the primary main switch control signal. FIG. 1 shows such an approach. There an input voltage source 2 supplies a series-connected primary winding 6 of a transformer 4 and a primary switch S1. A control signal Vc(S1) for the switch S1 is constant frequency control signal having a variable ON duty-cycle to assure a stable output voltage. To the output circuit, a secondary winding 8 of the transformer 4 provides a voltage 10 that alternates in polarity. The synchronous rectifier S2 couples an output load circuit comprising a load 24 and a filtering capacitor 22 to the output voltage 10 during the OFF time of S1's switching period. A synchronous rectifier S2 receives a control signal Vc(S2) through an inverter 26. Because of its effect during operation of the circuit, the body diode 18 of the MOSFET that is used as a synchronous rectifier is also shown in FIG. 1.
FIG. 1A displays the voltages and currents versus time waveforms for the flyback circuit parameters, i.e., Vc(S1), the control signal for the switch S1; Vc(S2), the control signal for switch S2 or synchronous rectifier; I(S1), the current through the switch S1; I(S2), the current through the switch S2 and V(S2), the voltage across the switch S2, for continuous-conduction mode (CCM). FIG. 1B displays the voltages and currents versus time waveforms for the discontinuous-conduction mode of operation (DCM). FIG. 1C depicts the same waveforms for the critical conduction mode of operation, the limit case between continuous mode and discontinuous mode of operation.
The main drawback of the above presented approach is the cross-conduction (concurrent conduction) of the primary switch S1 and secondary synchronous rectifier S2 for the time intervals t0-t1, t2-t3 (see FIG. 1A), for CCM operation. As a result, during these time intervals when the body-diode 18, an intrinsic part of the synchronous rectifier S2, is ON, supplementary power losses appear and have to be taken into account. Another drawback is related to power loss introduced by the reverse recovery, during t3-t4, of the body-diode 18 during turn-off of the primary switch S1, which adds to the general power balance or total power use of the circuit.
For DCM operation, See FIG. 1B, the power loss due to reverse recovery process of the body-diode 18 is eliminated because the current of the synchronous rectifier S2 becomes zero before the primary switch S1 is turned “on,” but the cross-conduction process during turn-off of switch S1 still exists. There are two drawbacks related with this mode of operation: higher conduction loss during Td (ON), see FIG. 1C, due to cross-conduction of I(S1) and I(S2) and the fact that the circuit efficiency fluctuates with Vin of the input voltage source 2.
The critical conduction mode of operation offers in certain conditions a zero voltage switching or ZVS feature which can be used if a proper design of the circuit is made. In this case the recover loss is eliminated also but the higher conduction loss associated with cross-conduction of I(S1) and I(S2), during Td(ON) delay, is still important.
There is a need therefore for a DC-DC flyback converter having a synchronous rectifier in its secondary circuit that is controlled such that cross-conduction losses are eliminated or substantially eliminated and reverse recovery losses are eliminated or substantially eliminated.